The 25-year-old told the club's official website: "I am very happy to sign for Chelsea. Everybody knows it is a big club in a very competitive league, and I am very excited to get started in England with a fantastic coach and teammates.

"Having played against Chelsea last season I know the high quality of the squad I am joining.

"I would like to thank everybody at Atletico who made me into the player I am, it was an incredible time for me, but now I am starting a new adventure and I hope to win many trophies with Chelsea."

Costa joins former Arsenal and Barcelona midfielder Cesc Fabregas in moving from the Primera Division to Stamford Bridge this summer.

His goals helped Atletico claim a first Spanish league title in 18 years last season and he helped the Madrid side to the Champions League final, beating Chelsea in the semifinal en route to Lisbon, where Real Madrid won the European Cup.